Guadalupe and Almaden reservoirs

The Guadalupe and Almaden reservoirs are among the original reservoirs in Santa Clara County constructed during the 1930s by the Santa Clara Valley Water Conservation District to save the maximum amount of water and boost the economy during the Depression.

More than $2 million in federal funds and public bonds built these two reservoirs, along with Calero, Coyote, Stevens Creek, Vasona, Anderson and Lexington. But only Almaden and Guadalupe share the same critical problem of extensive mercury contamination.

Almaden Reservoir sits only one mile away from the New Almaden Mines, once one of the largest mercury producing mine in the Americas. Guadalupe Reservoir is about two miles from the now closed mercury mines. Both reservoirs still feel the impacts of the old work, particularly during large runoff events when mercury containing sediments from mine wastes get into the water from the mining areas. Both are classified as an impaired body because of their level of toxic mercury.

What’s being doneSince 2003, the water district has evaluated the factors that may control the conversion of mercury to methylmercury. It installed solar powered circulators at both reservoirs to determine if improving dissolved oxygen in the water affects methylmercury production. So far, though, the circulators have not improved the reservoirs’ water quality. The district also examined the elevated sources of mercury in sediment entering the reservoirs and may install pilot oxygenation systems to see if this improves the situation. The Jacques Gulch project did remove the only source of mine waste to Almaden.

The water district will continue to treat the reservoirs as long as they remain in use. However, there is no long-term or permanent solution to deal with the problem. No one has attempted the best treatment alternative identified so far – oxygenation – and even if it does work, it will take several years for the results to appear in the fish that are of the size consumed by humans. The district will continue to evaluate treatment options and may find some opportunities to improve the reservoirs when it retrofits them for seismic reasons.